LIGHT EMITTING DIODE

• LEDs have replaced incandescent lamps in many applications because of the LED’s
• lower energy consumption,
• smaller size,
• faster switching and
• longer lifetime
Parts of an LED
Typical LEDs
Basic Circuit and Practical Circuit
• The brightness of an LED depends on the current. The amount of light emitted is
often specified as its luminous intensity (Iv) and is rated in candelas (cd).
• Low-power LEDs generally have their ratings given in millicandelas (mcd).
• For instance, a TLDR5400 is a red LED with a forward voltage drop of 1.8 V and an
Iv rating of 70 mcd at 20 mA.
High-Power LEDs
BIPOLAR JUNCTION TRANSISTOR
TRANSISTOR
• In 1951, William Schockley invented the first junction
transistor, a semiconductor device that can amplify
(enlarge) electronic signals such as radio and
television signals. The transistor has led to many other
semiconductor inventions, including the integrated
circuit (IC), a small device that contains thousands of
miniaturized transistors. Because of the IC, modern
computers and other electronic miracles are possible.
Bipolar Junction Transistor
BJT Structure

The BJT has three regions called the emitter, base, and
collector. Between the regions are junctions as indicated.
C (collector) C

n Base-Collector p
junction
B p B n
(base) Base-Emitter
n p
junction

E (emitter) E
Basic epitaxial planar structure
Bipolar Junction Transistor
BJT Structure

The BJT has three regions called the emitter, base, and
collector. Between the regions are junctions as indicated.

C (collector) C
The base is a thin
lightly doped region
compared to the n Base-Collector p
heavily doped emitter B
junction
p B n
and moderately doped (base) Base-Emitter
collector regions. n p
junction

pnp
npn

E (emitter) E
Bipolar Junction Transistor
Standard BJT Symbol
Bipolar Junction Transistor
Standard BJT Symbol

In summary, it is important to note the following points about the construction

of a transistor.
1. The emitter region is heavily doped. Its job is to emit or inject current carriers
into the base region. For npn transistors, the n -type emitter injects free
electrons into the base. For pnp transistors, the p –type emitter injects holes
into the base.
2. The base is very thin and lightly doped. Most of the current carriers injected
into the base region cross over into the collector side and do not flow out the
base lead.
3. The collector region is moderately doped. It is also the largest region within
the transistor. Its function is to collect or attract current carriers injected into the
base region.
Bipolar Junction Transistor
BJT Operation

In normal operation, the base-emitter is forward-biased

and the base-collector is reverse-biased.
For the npn type, the
collector is more positive BC reverse-
than the base, which is more biased
positive than the emitter.
+
+
–
+ –
+ –
BE forward-
–
biased
Bipolar Junction Transistor
BJT Operation

In normal operation, the base-emitter is forward-biased

and the base-collector is reverse-biased.
For the pnp type, the voltages
are reversed to maintain the
forward-reverse bias. BC reverse-
biased
–
+ –
–
– +
+
BE forward-
+ biased
• Example
• Example
• Example
Bipolar Junction Transistor
Transistor DC Bias Circuits
Transistor biasing for the common-base connection.

Proper biasing for an npn transistor. The EB junction is forward-biased by the emitter supply
voltage, VEE . VCC reverse-biases the CB junction.

Currents in a transistor.
Bipolar Junction Transistor
BJT Characteristics
• Example
 Another way to connect external voltages to the npn transistor.

VBB provides the forward bias for the

base-emitter junction, and VCC provides
the reverse bias for the collector-base
junction.

This connection is called the common-

emitter(CE) connection since the emitter
lead is common to both the input and
output sides of the circuit.
• Example
• Example
Bipolar Junction Transistor
BJT Characteristics

Transistor DC Model
Bipolar Junction Transistor
BJT Characteristics

BJT Circuit Analysis

Bipolar Junction Transistor
BJT Characteristics
Bipolar Junction Transistor
BJT Characteristics
• Example
Determine I , I , I , V , V , and V in the circuit The transistor has a b = 150.
B C E BE CE CB DC
Bipolar Junction Transistor
BJT Characteristics

The collector characteristic curves show the relationship

of the three transistor currents. The curve shown is for a
fixed based current. IC

C
B

A
VCE
0 0.7 V VCE(max)
Bipolar Junction Transistor
BJT Characteristics

The collector characteristic curves show the relationship

of the three transistor currents. The curve shown is for a
fixed based current I C

The first region from A to B is

the saturation region. As VCE is C
B
increased, IC increases until B.

Saturation
region

A
VCE
0 0.7 V VCE(max)
Bipolar Junction Transistor
BJT Characteristics

The collector characteristic curves show the relationship

of the three transistor currents. The curve shown is for a
fixed based current I C

The first region from A to B is

the saturation region. As VCE is Active region
C
B
increased, IC increases until B.
After reaching B, the curve
flattens between points B and C, Saturation
which is the active region. region

A
VCE
0 0.7 V VCE(max)
Bipolar Junction Transistor
BJT Characteristics

The collector characteristic curves show the relationship

of the three transistor currents. The curve shown is for a
fixed based current I C
Breakdown
The first region from A to B is region
the saturation region. As VCE is Active region
C
B
increased, IC increases until B.
After reaching B, the curve
flattens between points B and C, Saturation
which is the active region. region

After C, is the breakdown A

VCE
0 0.7 V VCE(max)
region.
Bipolar Junction Transistor
BJT Characteristics

The collector characteristic curves illustrate the relationship of the

three transistor currents. I C

By setting up other values of I B6

base current, a family of I B5

collector curves is developed.
I B4
bDC is the ratio of collector
I B3
current to base current.
I B2
I
b DC = C I B1
IB
Cutoff region IB = 0
It can be read from the curves. VCE

The value of bDC is nearly the

0

same wherever it is read.

Bipolar Junction Transistor
BJT Characteristics - Example

What is the bDC for the transistor shown?

I C (mA)

IB6 = 60 mA
10.0

IB5 = 50 mA
8.0
IB4 = 40 mA

6.0
I B3 = 30 mA

4.0 IB2 = 20 mA

IB1 = 10 mA
2.0

IB = 0
0 VCE
Bipolar Junction Transistor
BJT Characteristics - Example

What is the bDC for the transistor shown?

I C (mA)

IB6 = 60 mA
10.0
Choose a base current near the IB5 = 50 mA
center of the range – in this 8.0
IB4 = 40 mA
case IB3 which is 30 mA.
6.0
I B3 = 30 mA

4.0 IB2 = 20 mA

IB1 = 10 mA
2.0

IB = 0
0 VCE
Bipolar Junction Transistor
BJT Characteristics - Example

What is the bDC for the transistor shown?

I C (mA)

IB6 = 60 mA
10.0
Choose a base current near the IB5 = 50 mA
center of the range – in this 8.0
IB4 = 40 mA
case IB3 which is 30 mA.
6.0
I B3 = 30 mA
Read the corresponding
IB2 = 20 mA
collector current – in this case, 4.0

IB1 = 10 mA
5.0 mA. Calculate the ratio of 2.0

collector current to base current. IB = 0

0 VCE
Bipolar Junction Transistor
BJT Characteristics - Example

What is the bDC for the transistor shown?

I C (mA)

IB6 = 60 mA
10.0
Choose a base current near the IB5 = 50 mA
center of the range – in this 8.0
IB4 = 40 mA
case IB3 which is 30 mA.
6.0
I B3 = 30 mA
Read the corresponding
IB2 = 20 mA
collector current – in this case, 4.0

IB1 = 10 mA
5.0 mA. Calculate the ratio of 2.0

collector current to base current. IB = 0

0 VCE
I C 5.0 mA
bDC = = =
I B 30 m A
Bipolar Junction Transistor
BJT Characteristics - Example

What is the bDC for the transistor shown?

I C (mA)

IB6 = 60 mA
10.0
Choose a base current near the IB5 = 50 mA
center of the range – in this 8.0
IB4 = 40 mA
case IB3 which is 30 mA.
6.0
I B3 = 30 mA
Read the corresponding
IB2 = 20 mA
collector current – in this case, 4.0

IB1 = 10 mA
5.0 mA. Calculate the ratio of 2.0

collector current to base current. IB = 0

0 VCE
I C 5.0 mA
bDC = = = 167
I B 30 m A
Bipolar Junction Transistor
BJT Cutoff

In a BJT, cutoff is the condition in which there is no base

current, which results in only an extremely small leakage
current (ICEO) in the collector circuit. For practical work, this
current is assumed to be zero.
RC

ICEO
RB + +
VCE ≅ VCC VCC
IB = 0 –
–
Bipolar Junction Transistor
BJT Cutoff

In a BJT, cutoff is the condition in which there is no base

current, which results in only an extremely small leakage
current (ICEO) in the collector circuit. For practical work, this
current is assumed to be zero.
RC
In cutoff, neither the base-emitter
junction, nor the base-collector
junction are forward-biased. ICEO
RB + +
VCE ≅ VCC VCC
IB = 0 –
–
Bipolar Junction Transistor
BJT Saturation

In a BJT, saturation is the condition in which there is

maximum collector current. The saturation current is
determined by the external circuit (VCC and RC in this case)
because the collector-emitter voltage is minimum (≈ 0.2 V)
RC
– +

IC
RB + +
VCE = VCC – IC RC VCC
+ IB – –
VBB
–
Bipolar Junction Transistor
BJT Saturation

In a BJT, saturation is the condition in which there is

maximum collector current. The saturation current is
determined by the external circuit (VCC and RC in this case)
because the collector-emitter voltage is minimum (≈ 0.2 V)
RC
In saturation, an increase of base – +
current has no effect on the
IC
collector circuit and the relation
RB + +
IC = bDCIB is no longer valid.
VCE = VCC – IC RC VCC
+ IB – –
VBB
–
Bipolar Junction Transistor
BJT Load Line

The DC load line represents the circuit that is external to

the transistor. It is drawn by IC
connecting the saturation
and cutoff points. Saturation
IC(sat)

IB = 0 Cutoff
V CE
0 VCE(sat) VCC
Bipolar Junction Transistor
BJT Load Line

The DC load line represents the circuit that is external to

the transistor. It is drawn by IC
connecting the saturation
and cutoff points. Saturation
IC(sat)

The transistor characteristic

curves are shown superimposed
on the load line. The region
between the saturation and
cutoff points is called the
active region.

IB = 0 Cutoff
V CE
0 VCE(sat) VCC
Bipolar Junction Transistor
BJT Load Line - Example

What is the saturation current and RC 3.3 kW

the cutoff voltage for the circuit? RB +
Assume VCE = 0.2 V in saturation. VCC
βDC = 200
15 V
+ 220 kW –
V BB
3V –
Bipolar Junction Transistor
BJT Load Line - Example

What is the saturation current and RC 3.3 kW

the cutoff voltage for the circuit? RB +
Assume VCE = 0.2 V in saturation. VCC
βDC = 200
15 V
+ 220 kW –
V BB
VCC − 0.2 V 15 V − 0.2 V 3V –
ISAT = = =
RC 3.3 kW
Bipolar Junction Transistor
BJT Load Line - Example

RC 3.3 kW

What is the saturation current and RB +

VCC
βDC = 200
the cutoff voltage for the circuit? + 220 kW –
15 V

Assume VCE = 0.2 V in saturation. V BB

3V –

VCC − 0.2 V 15 V − 0.2 V

ISAT = = = 4.48 mA VCO = VCC = 15 V
RC 3.3 kW
Bipolar Junction Transistor
BJT Load Line - Example

RC 3.3 kW

What is the saturation current and RB +

VCC
βDC = 200
the cutoff voltage for the circuit? + 220 kW –
15 V

Assume VCE = 0.2 V in saturation. V BB

3V –

VCC − 0.2 V 15 V − 0.2 V

ISAT = = = 4.48 mA VCO = VCC = 15 V
RC 3.3 kW

Is the transistor saturated?

Bipolar Junction Transistor
BJT Load Line - Example

RC 3.3 kW

What is the saturation current and RB +

VCC
βDC = 200
the cutoff voltage for the circuit? + 220 kW –
15 V

Assume VCE = 0.2 V in saturation. V BB

3V –

VCC − 0.2 V 15 V − 0.2 V

ISAT = = = 4.48 mA VCO = VCC = 15 V
RC 3.3 kW

3.0 V − 0.7 V
Is the transistor saturated? I B = = 10.45 m A
220 kW
Bipolar Junction Transistor
BJT Load Line - Example

RC 3.3 kW

What is the saturation current and RB +

VCC
βDC = 200
the cutoff voltage for the circuit? + 220 kW –
15 V

Assume VCE = 0.2 V in saturation. V BB

3V –

VCC − 0.2 V 15 V − 0.2 V

ISAT = = = 4.48 mA VCO = VCC = 15 V
RC 3.3 kW

3.0 V − 0.7 V
Is the transistor saturated? I B = = 10.45 m A
220 kW
IC = b IB = 200 (10.45 mA) =
Bipolar Junction Transistor
BJT Load Line - Example

RC 3.3 kW

What is the saturation current and RB +

VCC
βDC = 200
the cutoff voltage for the circuit? + 220 kW –
15 V

Assume VCE = 0.2 V in saturation. V BB

3V –

VCC − 0.2 V 15 V − 0.2 V

ISAT = = = 4.48 mA VCO = VCC = 15 V
RC 3.3 kW

3.0 V − 0.7 V
Is the transistor saturated? I B = = 10.45 m A
220 kW
IC = b IB = 200 (10.45 mA) = 2.09 mA Since IC < ISAT, it is not saturated.
Bipolar Junction Transistor
BJT Data Sheets

Data sheets give manufacturer’s specifications for maximum operating

conditions, thermal, and electrical characteristics. For example, an
electrical characteristic is bDC, which is given as hFE. The 2N3904 shows
a range of b’s on the data sheet from 100 to 300 for IC = 10 mA.
Characteristic Symbol Min Max Unit
ON Characteristics
DC current g ain hFE –
( IC = 0.1 mA dc, VCE = 1.0 V dc) 2N3903 20 –
2N3904 40 –
( IC = 1.0 mA dc, VCE = 1.0 V dc) 2N3903 35 –
2N3904 70 –
( IC = 10 mA dc, VCE = 1.0 V dc) 2N3903 50 150
2N3904 100 300
( IC = 50 mA dc, VCE = 1.0 V dc) 2N3903 30 –
2N3904 60 –
( IC = 100 mA dc, VCE = 1.0 V dc) 2N3903 15 –
2N3904 30 –
Bipolar Junction Transistor
BJT Data Sheets

Data sheets give manufacturer’s specifications for maximum operating

conditions, thermal, and electrical characteristics. For example, an
electrical characteristic is bDC, which is given as hFE. The 2N3904 shows
a range of b’s on the data sheet from 100 to 300 for IC = 10 mA.
Characteristic Symbol Min Max Unit
ON Characteristics
DC current g ain hFE –
( IC = 0.1 mA dc, VCE = 1.0 V dc) 2N3903 20 –
2N3904 40 –
( IC = 1.0 mA dc, VCE = 1.0 V dc) 2N3903 35 –
2N3904 70 –
( IC = 10 mA dc, VCE = 1.0 V dc) 2N3903 50 150
2N3904 100 300
( IC = 50 mA dc, VCE = 1.0 V dc) 2N3903 30 –
2N3904 60 –
( IC = 100 mA dc, VCE = 1.0 V dc) 2N3903 15 –
2N3904 30 –

courccontent
Bipolar Junction Transistor
DC and AC Quantities

The text uses capital letters for both AC and DC currents and voltages
with rms values assumed unless stated otherwise.
DC Quantities use upper case roman subscripts. Example: VCE.
(The second letter in the subscript indicates the reference point.)
Bipolar Junction Transistor
DC and AC Quantities

The text uses capital letters for both AC and DC currents and voltages
with rms values assumed unless stated otherwise.
DC Quantities use upper case roman subscripts. Example: VCE.
(The second letter in the subscript indicates the reference point.)
AC Quantities and time varying signals use lower case italic
subscripts. Example: Vce.
Bipolar Junction Transistor
DC and AC Quantities

The text uses capital letters for both AC and DC currents and voltages
with rms values assumed unless stated otherwise.
DC Quantities use upper case roman subscripts. Example: VCE.
(The second letter in the subscript indicates the reference point.)
AC Quantities and time varying signals use lower case italic
subscripts. Example: Vce.
Internal transistor resistances are indicated as lower case
quantities with a prime and an appropriate subscript. Example: re’.
Bipolar Junction Transistor
DC and AC Quantities

The text uses capital letters for both AC and DC currents and voltages
with rms values assumed unless stated otherwise.
DC Quantities use upper case roman subscripts. Example: VCE.
(The second letter in the subscript indicates the reference point.)
AC Quantities and time varying signals use lower case italic
subscripts. Example: Vce.
Internal transistor resistances are indicated as lower case
quantities with a prime and an appropriate subscript. Example: re’.
External resistances are indicated as capital R with either a
capital or lower case subscript depending on if it is a DC or ac
resistance. Examples: RC and Rc.
Bipolar Junction Transistor
BJT Amplifiers

A BJT amplifies AC signals by converting some of the DC power from

the power supplies to AC signal power. An ac signal at the input is
superimposed in the dc bias by the capacitive coupling. The output ac
signal is inverted and rides on a dc level of VCE.
RC
Vin

VBB
RB +
0 r e′ VCC
Vc Vc –
+
Vin VBB Vb
–
VCE

0
Bipolar Junction Transistor
BJT Switches

A BJT can be used as a switching device in logic circuits to turn on or

off current to a load. As a switch, the transistor is normally in either
cutoff (load is OFF) or saturation (load is ON).

+ VCC +VCC +VCC +VCC

RC IC = 0 RC RC IC(sat) RC IC(sat)

RB C RB C
+
0V +VBB
IB = 0 E
E IB –

In cutoff, the transistor In saturation, the transistor

looks like an open switch. looks like a closed switch.
Bipolar Junction Transistor
BJT Switches

A BJT can be used as a switching device in logic circuits to turn on or

off current to a load. As a switch, the transistor is normally in either
cutoff (load is OFF) or saturation (load is ON).
+ VCC +VCC +VCC +VCC

RC IC = 0 RC RC IC(sat) RC IC(sat)

RB C RB C
+
0V +VBB
IB = 0 E IB – E

In cutoff, the transistor

looks like an open switch. In saturation, the transistor
looks like a closed switch.